sch2fig/: implement "diff" driver (WIP)

sch2fig/TODO

@@ -19,3 +19,8 @@
 - on parse error, politely complain, don't terminate
 - implement destructors
 - check for memory leaks
+- record.c (bb_rot): implement bounding boxes for text
+- nesting gfx in diff is a huge kludge, caused by global vars in gfx.c
+- move path name guessing into file.c
+- return indication of whether diff found any differences
+- in diff, pass only options understood by cairo_png